1. Field of the Invention
The present invention relates to an image display apparatus using a hold-type display device such as, for example, a liquid crystal display device or an EL (electroluminescence) display device; an electronic apparatus, a liquid crystal TV, a liquid crystal monitoring apparatus, which use such an image display apparatus for a display section; an image display method performing image display using such an image display apparatus; a display control program for allowing a computer to execute the image display method; and a computer-readable recording medium having the display control program recorded thereon.
2. Description of the Related Art
Conventional image display apparatuses are roughly classified into impulse-type display apparatuses such as CRTs (cathode ray tubes), film projectors and the like; and hold-type display apparatuses using hold-type display devices such as liquid crystal display devices, EL display devices and the like mentioned above.
In impulse-type display apparatuses, a light-on period in which an image is displayed and a light-off period in which no image is displayed are alternately repeated. It is considered that human eyes perceive, as the brightness, a luminance obtained by time integration of a luminance change of an image which is actually displayed on the screen during a period of about several frames. Therefore, human eyes can observe, with no unnatural feeling, an image displayed by an image display apparatus, such an impulse-type image display apparatus, in which the luminance changes within a short period of one frame or less.
FIG. 46 shows a luminance change in accordance with time of one horizontal line in a screen when an object horizontally moves with a still background in a conventional impulse-type image display apparatus. In FIG. 46, the horizontal axis represents the luminance state in the horizontal direction of the screen (the position of the pixel portion in the horizontal direction), and the vertical axis represents the time. FIG. 46 shows images displayed on the screen in three frames.
In FIG. 46, each one-frame period T101 is a cycle by which the image is updated. In the impulse-type image display apparatus shown in FIG. 46, a light-on period T102 is at the beginning of each one-frame period T101. A light-off period T103 follows the light-on period T102 until the image is updated in the next frame. In the light-off period T103, the luminance is minimum.
Regarding the display state of one horizontal line, a display portion A of the moving object is sandwiched between display portions B of the still background. Each time the image is updated frame by frame, the display portion A moves rightward.
The observer's eye paying attention to the display portion A follows the display portion A and thus moves in the direction represented by the oblique thick arrow. A value obtained by time integration of a luminance change in the direction of the movement of the object is perceived as the brightness by the human eye.
FIG. 47 shows the distribution in brightness of the image shown in FIG. 46 which is viewed by the observer's eye paying attention to the moving object.
In the case of the impulse-type image display apparatus, the period from an image update to the next image update is mostly a light-off period T103. The luminance in the light-off period T103, which is sufficiently low, does not contribute to the time-integrated luminance (value of the vertical axis). As a result, the observer's eye clearly views the difference in brightness at the border between the still background and the moving object. Therefore, the observer's eye can clearly distinguish the object from the background.
It is considered that hold-type image display apparatuses are inferior to the impulse-type image display apparatuses in the quality of moving images. This will be described in detail below.
FIG. 48 shows a luminance change in accordance with time of one horizontal line in a screen when an object horizontally moves with a still background in a general conventional hold-type image display apparatus. In FIG. 48, the horizontal axis represents the luminance state in the horizontal direction of the screen (the position of the pixel portion in the horizontal direction), and the vertical axis represents the time. FIG. 48 shows images displayed on the screen in three frames.
In FIG. 48, unlike in FIG. 46, each one-frame period T101 is entirely a light-on period T102. No light-off period is provided.
FIG. 49 shows the distribution in brightness of the image shown in FIG. 48 which is viewed by the observer's eye paying attention to the moving object.
Since the one-frame period T101 is entirely a light-on period T102, the object is displayed as remaining at the same position from an image update until the next image update. As a result, the value obtained by time integration of a luminance change in the direction of the movement of the object does not reflect the difference in brightness at the border between the still background and the moving object. Therefore, the observers eye views the border as a movement blur. This is one cause of the poor image quality of general conventional hold-type image display apparatuses.
One solution to this problem of the hold-type image display apparatuses is to reduce the duration of the light-on period to about half and provide a period in which image display is performed at the minimum luminance level (minimum luminance period). Hereinafter, this system will be referred to as the “minimum (luminance) insertion system”.
FIG. 50 shows a luminance change in accordance with time of one horizontal line in a screen when an object horizontally moves with a still background in a conventional hold-type image display apparatus which adopts the minimum (luminance) insertion system. In FIG. 50, the horizontal axis represents the luminance state in the horizontal direction of the screen (the position of the pixel portion in the horizontal direction), and the vertical axis represents the time. FIG. 50 shows images displayed on the screen in three frames.
In FIG. 50, unlike in FIG. 48, each one-frame period T101 includes a ½-frame light-off period (or a minimum luminance period or a minimum (luminance) insertion period) T103.
FIG. 51 shows the distribution in brightness of the image shown in FIG. 50 which is viewed by the observer's eye paying attention to the moving object.
FIG. 51 shows that the movement blur is alleviated, as compared with the general conventional hold-type image display apparatus shown in FIG. 49.
However, in the conventional hold-type image display apparatus which adopts the minimum (luminance) insertion system, each one-frame period includes a minimum luminance period (or a minimum (luminance) insertion period or a light-off period) even when the image display is performed at the maximum gradation level. Therefore, the maximum luminance perceived by the observer's eye is half of that in the general conventional hold-type image display apparatuses which do not adopt the minimum (luminance) insertion system.
Especially when a display device, such as an EL display device, which spontaneously emits light, is used for such a hold-type image display apparatus, the reduction in the maximum luminance is inevitable as compared with the general conventional hold-type image display apparatuses which do not adopt the minimum (luminance) insertion system.
Another solution to the problem of movement blur has been proposed for transmissive display devices such as transmissive liquid crystal display devices and the like. According to the proposed solution, the luminance of the backlight is increased in order to guarantee approximately the same level of maximum luminance as that of the general conventional hold-type image display apparatuses which do not adopt the minimum (luminance) insertion system.
This proposed solution has the following drawbacks. First, the power consumption of the backlight is raised. Second, even while the image display is performed at the minimum luminance (black period), the light from the backlight can be transmitted through the display device. Therefore, the minimum luminance level cannot be approximately the same as that of the hold-type image display apparatuses which do not adopt the minimum (luminance) insertion system. As a result, the contrast is reduced.
Japanese Laid-Open Publication No. 2001-296841 proposes the following image display method by claims 27 through 41 in order to improve the quality of moving images by, for example, solving the problem of movement blur while guaranteeing approximately the same level of maximum luminance as that of the general conventional hold-type image display apparatuses which do not adopt the minimum (luminance) insertion system. A specific method for driving the display device and providing an image signal of a certain gradation level is described in example 7 of Japanese Laid-Open Publication No. 2001-296841 in detail. Japanese Laid-Open Publication No. 2001-296841 is entirely incorporated herein for reference.
According to the image display method proposed by Japanese Laid-Open Publication No. 2001-296841, one frame of image display is performed using two sub frame periods, i.e., the first sub frame period and the second sub frame period. When the gradation level of an input image signal is 0% or greater and less than 50%, an image signal of a gradation level of 0% to 100% is supplied in the first sub frame period, and an image signal of a gradation level of 0% is supplied in the second sub frame period. When the gradation level of the input image signal is 50% or greater and less than 100%, an image signal of a gradation level of 0% to 100% is supplied in the first sub frame period, and an image signal of a gradation level of 100% is supplied in the second sub frame period.
FIG. 52 shows a luminance change in accordance with time of one horizontal line in a screen when an object horizontally moves with a still background in a conventional hold-type image display apparatus disclosed by Japanese Laid-Open Publication No. 2001-296841. In FIG. 52, the horizontal axis represents the luminance state in the horizontal direction of the screen (the position of the pixel portion in the horizontal direction), and the vertical axis represents the time. FIG. 52 shows images displayed on the screen in three frames.
In FIG. 52, unlike in FIG. 48, each one-frame period T101 includes two sub frame periods T201 and T202.
This will be described in more detail. As shown in FIG. 52, for a display portion B of the still background, the gradation level of an input image signal is low. Therefore, the display portion B is in a light-on state only in the first sub frame period T201 and is in a light-off state (0%) in the second sub frame period T202. For a display portion A of the moving object, the gradation level of the input image signal is sufficiently high. Therefore, the display portion A is in a light-on state at the maximum luminance (100%) in the second sub frame period T202, and is in a light-on state at the luminance of 20% with an image signal of a gradation signal of 0% to 100% in the first sub frame period T201. The numerals with “%” represent the luminance level of the image with respect to the maximum display ability of 100%. For example, the numeral surrounded by the dotted line for B1 represents the luminance of 40%.
Such an image display method can guarantee approximately the same level of maximum luminance and contrast as those of the conventional hold-type image display apparatuses which do not adopt the minimum (luminance) insertion system, and also can improve the quality of moving images where the gradation level of the input image signal is sufficiently low.
Japanese Laid-Open Publication No. 2002-23707 discloses another method for suppressing the reduction in luminance of the hold-type image display apparatuses which adopt the minimum (luminance) insertion system. According to the method disclosed by Japanese Laid-Open Publication No. 2002-23707, a one-frame period includes a plurality of sub frame periods, and the luminance of one of the latter frames is attenuated at a prescribed ratio in accordance with the luminance of an input image signal. Therefore, the movement blur which is visually perceived in the general conventional hold-type image display apparatuses can be prevented. Since the luminance of one of the latter sub frame periods is attenuated as described above and thus is not 0%, the reduction in luminance can be suppressed as compared with the conventional hold-type image display apparatuses which adopt the minimum (luminance) insertion system as shown in FIGS. 50 and 51.
For displaying an image of an object moving horizontally with a still background, the conventional image display apparatus disclosed by Japanese Laid-Open Publication No. 2001-296841 can provide substantially the same effect as that of the conventional hold-type image display apparatus which adopts the minimum (luminance) insertion system shown in FIGS. 50 and 51, as long as the gradation level of the input image signal is sufficiently low. However, when the gradation level of the input image signal is high, the following problems occur.
FIG. 53 shows the distribution in brightness of the image shown in FIG. 52 which is viewed by the observer's eye paying attention to the moving object.
As shown in FIG. 53, a portion of the image is brighter than the original image and another portion of the image is darker than the original image. As a result, the observer's eye views abnormally bright and abnormally dark portions at the leading end or the trailing end of the moving object, which are not viewed in a still image. This lowers the quality of moving images.
The reason why such abnormally bright and abnormally dark portions are viewed is that the time-wise center of gravity of the light-on period is significantly different between when the gradation level of the input image signal is less than 50% and when the gradation level of the input image signal is 50% or greater. For example, when the gradation level of the input image signal is less than 50%, the time-wise center of gravity of luminance in the light-on period is the first sub frame period T201 since an image signal of a gradation level of 0% is supplied in the second sub frame period T202. When the gradation level of the input image signal is 50% or greater, the time-wise center of gravity of the light-on period (display luminance) is the second sub frame period T202 since an image signal of a gradation level of 100% is supplied in the second sub frame period T202. For this reason, abnormally bright and abnormally dark portions are viewed at the leading end or the trailing end of the moving object, in terms of the value obtained by time integration of a luminance change in the direction of the movement of the object.
Current general image signals, for example, TV broadcast signals, video reproduction signals, and PC (personal computer) image signals, are mostly generated and output in consideration of the gamma luminance characteristic of CRTs (cathode ray tubes). Display panels which use the hold-type display devices such as, for example, liquid crystal display devices and EL display devices generally have substantially the same gamma luminance characteristic as that of CRTs in order to be compatible with the general image signals.
FIG. 54 is a graph illustrating the relationship between the gradation level of an input image signal and the display luminance of a display panel having such a gamma luminance characteristic. As shown in FIG. 54, the relationship is represented by a curve which is generally concaved toward lower luminance. From this, it is understood that the point of luminance of 50% and the point of gradation level of 50% do not match each other.
FIG. 55 shows the relationship between the gradation level of an input signal and the time-integrated luminance corresponding to the brightness perceived by the observer's eye, when the display control as described in example 7 of Japanese Laid-Open Publication No. 2001-296841 is performed using a hold-type image display device having the gamma luminance characteristic.
In example 7 of Japanese Laid-Open Publication No. 2001-296841, when the gradation level of the input image signal is 50% or greater, an image signal is supplied in two sub frame periods (the first and second sub frame periods). By contrast, when the gradation level of the input image signal is less than 50%, an image signal is supplied in only one sub frame period (only in the first sub frame period). Therefore, the luminance characteristic curve has two concaves at the point of luminance of 50% in the center thereof. With such a luminance characteristic curve, an appropriate color reproducibility to a general input image signal cannot be realized.
The method disclosed by Japanese Laid-Open Publication 2002-23707 places the image into a light-on state in one of the latter sub frame periods of each one-frame period, and thus can suppress the reduction in luminance and contrast as compared with the general hold-type image display apparatus which adopt the minimum (luminance) insertion type shown in FIGS. 50 and 51. However, this method does not provide a significant effect for preventing the movement blur. In addition, the contrast obtained by this method is lower than that of the general conventional hold-type image display apparatuses.